Weld electrode for attractive weld appearance

ABSTRACT

Weld faces of electrodes for resistance spot welding are formed with a suitable area of protrusions and/or intrusions. The size, shape, and elevation or depths of the protrusions or intrusions are determined for the formation of suitable spot welds in and between metal workpieces such as aluminum or steel panels for vehicle bodies. The protrusions or intrusions are also conceived and used to form an image on at least a visible surface of the welded article to produce an attractive appearance on the surface of the welded sheet.

This application is a continuation-in-part of co-pending applicationSer. No. 12/251,635, filed Oct. 15, 2008 and titled, “Welding Electrodefor Attractive Weld Appearance, which, in turn, is acontinuation-in-part of co-pending application Ser. No. 11/536,001,filed Sep. 28, 2006 and titled, “Welding Electro de with Contoured Face.The text and drawings of co-pending application Ser. No. 11/536,001 areincorporated herein by reference.

TECHNICAL FIELD

This invention pertains to electrodes for the formation of electricalresistance spot welds between two or more sheet metal layers. Morespecifically this invention pertains to welding electrodes for formingof high quality weld nuggets between the metal layers and forsimultaneously forming a pleasing pattern or image at the position wherethe welding electrode contacts the surface.

BACKGROUND OF THE INVENTION

The above identified parent application describes welding electrodeswith a round weld face for contact with a metal workpiece in anelectrical resistance welding operation. The weld face includesconcentric rings of ridges and/or grooves extending radially from thecenter of the weld face; the ridges extending axially upwardly from theface and the grooves extending inwardly into the face. When the face ofthe electrode is pressed into contact with a surface of the metalworkpiece for delivery of a welding current, these shaped features onthe face of the electrode penetrated surface oxides or otherconductivity barriers to facilitate passage of a welding current.

Current automotive vehicle manufacturing operations include, forexample, the joining of two sheet metal layers by spot welding. Vehiclebody panels such as doors, hoods, deck lids and liftgates are oftenassembled by joining inner and outer panels stamped from sheet metal ofsuitable metal alloys. Ferrous or aluminum alloys are often used. Thethickness of each sheet metal layer may vary from less than onemillimeter to more than four millimeters. Electrical resistance spotwelding is often used to join such inner and outer panels or other metalparts. For example, an edge of an outer panel sheet may be folded over acomplementary edge of an inner panel sheet in an assembly of the panelsin which the hem(s) is at the periphery of the sheets. The panelassembly is positioned for welding in areas removed from the hem joint.Axially aligned and opposing electrodes are pressed toward each otheragainst opposite sides of the panel assembly. A momentary weldingcurrent is passed between the electrodes through the layers of metal toform a spot weld. The spot weld is characterized by a momentarily fusedpool of metal and a re-solidified weld nugget at the interface of thecontacting sheets. The electrodes are retracted and moved to anotherweld site. The welding electrodes of the parent application Ser. No.11/536,001 with concentric features on their welding faces form highquality welds in metal workpieces.

In some applications spot weld sites may be visible in the surface of afinished product such as an automotive vehicle body panel or member.Several types of joints are used to join sheet metal panels in areasthat may be visible to customers. These areas include joints formed onvehicle body closure members such as hoods, decklids, liftgates, anddoors. Visible joints in the vehicle structure also include those in theengine compartment and trunk. It has been observed that the weldingelectrodes of the parent application Ser. No. 11/536,001 not only formstrong durable spot welds in such welded articles, but the circularridges or grooves leave a faithful impression on a visible surface of aspot weld site. The clear visible ring pattern in the surface of thewelded assembly is found to be much more pleasing to a consumer than thetraditionally formed welding electrode impression. Such a pleasing weldappearance can also influence the perceived quality of the assembly,that is, the appearance details alone can influence the consumer'sperception as to whether a product w ill fulfill his or herexpectations. And where a progressive sequence of such weld sites isvisible, the suggestion of high quality is reinforced.

Resistance spot weld appearance can suffer from several undesirablefeatures when using current, conventional spot weld electrodes. Forexample, a domed electrode with a flat welding face machined in itscenter is widely used for spot welding steel. It tends to leave a sharpimprint on the sheet surface that can have excessive indentationespecially if the electrode engages the sheet surface in an off-normalorientation when forming a spot weld. The electrode shape is also proneto causing sheet deformation around the weld that is unattractive. Metalexpulsion from excessive heat and off-normal orientation can lead towhiskers or fingers of metal protruding from the sheet surface which isundesirable.

Such welding surface issues have not arisen when using the electrodes ofthe parent application Ser. No. 11/536,001. So it is an object of thisinvention to provide additional welding electrode face features thatassure the formation of high strength or structural spot welds inworkpieces, such as steel and aluminum sheet metal workpieces, whileleaving behind clear visible indicia in a weld surface that provides anattractive appearance

SUMMARY OF THE INVENTION

This invention is a method to improve resistance spot weld appearance byintroducing visible features on the joint's visible surface. Theappearance is controlled by using an electrode shape that deliberatelyimparts a selected design on (or into) the sheet surface by takingadvantage of the heat and pressure used during the spot weldingoperation. Shaped features are incorporated in at least one of theopposing electrode welding faces that contacts at least one of the outersheet metal surfaces. These shaped features on the electrode facecontribute to the formation of a suitable spot weld and leave animpressed image in the weld surface. The image may be formed usingeither a protrusion extending from the weld face surface, an intrusioninto the weld face surface, or a combination of protrusions andintrusions. The image may be in the form of lines, geometric figures,alphanumeric symbols, or other recognizable and distinguishable images,such as a trademarked logo. The formation of such a discernable imageprovides an attractive appearance, but is not necessarily indicative ofthe underlying weld strength.

In the making of resistance spot welds between overlying sheet layers aweld nugget is formed at the interface of the sheets. The diameter ofthe weld nugget is typically smaller than the diameter of the weld faceof the electrode so the image on the sheet surface at the weld site isnot limited to the size or precise location of the weld nugget.

Formation of the selected design during the welding process may differsomewhat depending on whether the welded material is an aluminum alloyor steel material. For the case of aluminum alloy material, theprotrusion/intrusion on the electrode face forms the design in thematerial surface by reforming the aluminum alloy surface under theelevated temperature and pressure that is produced during the spotwelding process. Aluminum alloys are good conductors of heat andelectrical current. These attributes produce a softened zone around theweld nugget and at the sheet surface which can be deformed into thedesired design. Since the softened zone extends well beyond the formednugget, the design can also extend well beyond the nugget perimeter.

Formation of the selected design in steel materials may be somewhatdifferent. The higher thermal and electrical resistivity of steel, alongwith its high melting point, makes it somewhat more difficult to form adesign in the sheet surface with commonly used copper electrodematerials. However, nearly all steel materials used within automotivevehicle bodies are coated with a layer of zinc or zinc-iron. During thetraditional spot welding process, this zinc layer is typically meltedearly in the welding process and displaced from beneath the electrode byhydraulic pressure. If an electrode is used with a design in thesurface, the molten zinc may be trapped within the design so that it isnot fully displaced from beneath the welding electrode. Provided thatthe electrode temperature does not exceed the boiling point of zinc orzinc-iron, once weld current is stopped, the molten zinc or zinc-ironre-solidifies taking the shape of the electrode surface. The design isthen incorporated by a combination of sheet deformation and “re-casting” of the surface zinc. Since the design is partially formed byrecasting zinc, which requires a low temperature to melt, the design canextend well beyond the nugget perimeter where sheet metal is colder andmore difficult to form.

In each welding application, the electrode face is sized and shaped toform a good weld at the interfacial surfaces of contacting sheetsurfaces, i.e., the faying interface. And the electrode faces are shapedto leave a corresponding pattern of intrusions and/or protrusions in thewelded surface that are intended to suggest to a viewer that the imagein the surface indicates that a predetermined and suitable high qualityweld has been formed. In order to produce an observable feature in thewelded surface(s) it is preferred that protrusions or intrusions on theelectrode faces have a minimum height/depth of about ten microns. As isdescribed below in this specification the protrusions/intrusions may belarger depending on the thicknesses of the workpieces to be welded.

In some embodiments of the invention the electrode face is round asviewed in the plan view of the face. Further, in some embodiments, theelectrode face has a spherical surface with a radius centered on theaxis of the electrode. In other embodiments, the circular weld face isoffset from the electrode body axis and even tilted relative to it. Inmany such embodiments, the center of the electrode face is the firstportion of the face to engage the surface of a workpiece to be welded.In some welding situations it may be preferred to locate intrusions orprotrusions a predetermined distance from the axial center of theelectrode face for the purpose of limiting excessive welding currentflow through the intrusions or protrusions.

Accordingly, one aspect of the present invention is in the design of theface of an electrical resistance welding electrode to affect good weldsand leave a desired surface image. And another aspect of the inventionis in the method of using such electrodes to form spot welds in metalworkpieces, particularly in surfaces of workpieces that are visible to auser of the welded article.

Other objects and advantages of the invention will be apparent from adescription of illustrative embodiments which follows in thisspecification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a side elevation, schematic view, partly in cross-section, ofthe weld face portions of a pair of opposing, axially aligned resistancewelding electrodes about to engage portions of two overlapping aluminumalloy sheets to form an electrical resistance spot weld. The electrodefaces have protrusions for forming a pattern of grooves in the engagedsurfaces of the aluminum sheets.

FIG. 1B shows the electrodes in a withdrawn position after they haveengaged the aluminum sheets and formed a weld nugget at the interface ofthe sheet surfaces. The outer surfaces of the aluminum sheets aredeformed by the protrusions on the electrode faces.

FIG. 2A is a side elevation, schematic view, partly in cross-section, ofthe weld face portions of a pair of opposing, axially aligned resistancewelding electrodes about to engage portions of two overlappingzinc-coated steel alloy sheets to form a resistance spot weld. Theelectrode faces have protrusions for forming grooves in the engagedsurfaces of the zinc-coated steel sheets.

FIG. 2B shows the electrodes engaging and penetrating through the zinccoating on the steel sheets and forming a weld nugget at the interfaceof the sheet surfaces. The outer surfaces of the steel sheets areslightly deformed by the protrusions on the electrode faces.

FIG. 3A illustrates in cross-section an unsuitable protrusion electrodeface shape with an entry angle such that the protrusion could becomejoined to a sheet surface. In this illustration the sheet surface is analuminum alloy sheet surface.

FIG. 3B is an enlarged fragmentary view of a protrusion formed on andextending from the electrode face of FIG. 3A. FIG. 3B illustratespreferred and non-preferred surfaces on the protrusion from the weldface.

FIG. 4 illustrates in cross-section an unsuitable intrusion electrodeface shape with an entry angle such that the intrusion could becomejoined to extruded material from a sheet surface. In this illustrationthe sheet surface is a zinc-coated steel alloy sheet surface.

FIG. 5 illustrates a surface of a workpiece in which the letters “GM”have been formed at weld sites by intrusions in the weld face of anelectrode.

FIG. 6 illustrates a protrusion or intrusion design for a weld face forforming the image of a snow flake or star in a welded surface.

FIG. 7 illustrates a protrusion design for a resistance weld electrodeface for forming the image of a Chevrolet “Bow Tie” registered trademarklogo in a welded surface. The actual protrusion design on the weldingelectrode would be in mirror image of the illustration of FIG. 7 so thatthe weld surface image would look like FIG. 7.

DESCRIPTION OF PREFERRED EMBODIMENTS

One or more protrusion and/or intrusion elements are conceived,determined, and formed on the weld face of an electrical resistance spotweld electrode. These elements serve to suitably engage a sheet surfaceto be welded to form a suitable spot weld nugget between the contactingsheet surfaces. But the elements also are shaped to provide anattractive appearance in a visible welded surface when viewed by anobserver.

Accordingly, the protrusions and/or intrusions must meet a combinationof requirements. First, excessive indentation of the sheet material isto be avoided. Excessive indentation or metal thinning may weaken theweld and/or surrounding sheet metal. For a weld consisting of two ormore sheets, indentation occurs on the outer surfaces of the twooutermost sheets. The amount of indentation is typically more importantfor the thinner of the two sheets, since any given level of indentationmakes up a larger portion of the thickness of the thinner sheet. In allcases of aluminum and steel welding, the indentation in the outer twosheet surfaces should not be greater than 50% of the sheet thickness.Preferably for aluminum spot welding, since aluminum is more notchsensitive than steel, the amount of indentation should not exceed 20% ofthe sheet thickness. In both cases, a preferred level of indentationwould be about 15% or less of the sheet thickness since this would havea minimal effect on weld durability. Even a 1% indentation of a typicalsheet thickness can leave a pattern that is visible to the human eye.

FIG. 1A is a fragmentary schematic illustration of the opposingelectrical resistance spot weld electrodes 10, 12 poised for engagementwith an assembly of overlapping aluminum sheets 14, 16. Groupings ofthree concentric circular protrusions 18 on the face 22 of electrode 10and three like concentric circular protrusions 20 on the face 24 ofelectrode 12 have been machined on the spherically curved welding faces22, 24. The height of the protrusions and depths of intrusions may beexaggerated in the drawing figures for viewing.

Electrodes for resistance spot welding are typically formed of copper orlow resistivity copper alloys. They typically have a round body with ashank portion that is secured in a weld gun mounted on a robot or otherwelding apparatus for conducting a suitable welding current to theelectrodes and for locating them in opposing axial alignment againstopposite outer faces of a sheet metal assembly in which one or morewelds are to be formed. As illustrated in FIG. 1A and several of theother drawing figures, the end of the electrodes 10, 12 may be tapered,for example in the shape of a truncated cone or a truncated sphere, to aflat or rounded welding face. In the illustrative embodiments of thisinvention the welding faces (22, 24 in FIGS. 1A and 1B) are sphericallyrounded, suitably with a radius of for example about 20 mm to about 50mm, for easier engagement with a less-then-perfectly aligned workpiecesurface.

The protrusions may be in the configuration of concentric circles,concentric squares or other geometric shapes. The protrusions may beparallel lines, or alphanumeric letters or numerals, or other meaningfulone dimensional or two dimensional indices. In the FIGS. 1A and 1Bembodiment of the invention, the protrusions 18, 20 are in the form ofthree concentric circles that extend outwardly from the rounded face 22,24 of each electrode 10, 12. But in some embodiments it may be desiredto employ the protrusions only on an electrode face that engages asurface of a workpiece that may be visible to a user of a finishedwelded article. The sides of the protrusions 18, 20 are preferablytapered toward the end (as illustrated in FIG. 1A and FIG. 1B) so thatthey may enter into a softened aluminum alloy surface without bonding toit (as is described in more detail below).

In FIG. 1B the electrode faces have engaged the outer surfaces of theoverlying aluminum alloy sheet workpieces to form a spot weld and thenbeen withdrawn. In forming the weld, a welding current was passedbetween the electrodes to heat a generally cylindrical path through theoverlying sheets. A molten metal pool is momentarily formed at theinterface of the sheets 14, 16 which loses heat to the surrounding metaland re-solidifies as a weld nugget 26 that contains metal from bothsheets 14, 16 and joins them at the welded spot. The rounded electrodefaces 22, 24 with their respective protrusions 18, 20 have deformed themomentarily heat softened outer aluminum sheet surfaces to form andleave attractive patterns 28, 30 of three concentric rings embossed inthe outer surfaces of the now spot welded workpieces 14, 16.

FIGS. 2A and 2B illustrate the use of like or similar opposing weldingelectrodes 110, 112 to form a spot weld in an assembly of overlyingzinc-coated steel sheet workpieces 114, 116 (with zinc coatings 115,117). FIG. 2A shows the axially aligned and opposing electrodes 110, 112with their rounded faces 122, 124 and tapered and rounded protrusions118, 120 ready to engage a predetermined welding site on the assembledsheets 114, 116. In FIG. 2B the electrodes 110, 112 are shown inengagement with the welding surfaces and having formed the weld nugget126 between the contacting sheets 114, 116. In this illustration therehas been less deformation of the steel sheets. But the zinc coatinglayers 115, 117 (which may be several micrometers thick) have beendeformed and recast in the moderately deformed steel surfaces to leaveembossed patterns in the respective outer surfaces of workpieces 114,116. As in the embodiment illustrated in FIG. 1B, the embossed patternswill substantially be the negative shapes of protrusions 118, 120.

A second requirement of the protrusion/intrusions is that they not causeexcessive sticking between the electrode and sheet stack-up. Excessivesticking can cause serious problems in production environments. FIGS.3A, 3B, and 4 illustrate examples where the protrusions and intrusionson an electrode welding face may cause the electrode to become joined tothe deformed sheet surface. FIGS. 3A and 3B illustrate a spot weldelectrode 210 with welding face 222 and its angled protrusions 218engaging and penetrating a surface of aluminum sheet workpiece 214.Protrusions 218 are not tapered and engage the surface of aluminum sheet218 at an angle displaced from a vertical angle. Similarly, in FIG. 4,spot weld electrode 310 is shown with welding face 322 and its angledintrusions 318 engaging a surface of a steel sheet 314 with its zinccoating layer 315. In this example workpiece metal enters the intrusions318 machined in the weld face 322 of electrode 310. While in spotwelding operations two electrodes and welding surfaces are typicallyinvolved, in both FIGS. 3A and 4, a single welding electrode and sheetare shown to illustrate the angles of engagement of the respectiveprotrusions or intrusions with a workpiece surface.

The shape of the straight-sided protrusions (e.g., 218 in FIGS. 3A and3B) or intrusions (e.g., 318 in FIG. 4) produce contact angles with theworkpieces 214 and 314, 315 that tend to lock the electrode face to theworkpiece surface. In both cases the angle(s) between the exteriorsurface(s) of the sidewalls of the protrusion/intrusion and the plane ofthe sheet surface must be controlled such that 1) protrusions do notbecome irreversibly imbedded in the sheet surface and 2) intrusions donot result in the entrapment of deformed sheet metal and/or molten zinc.To accomplish this, the electrode face geometry should be designed suchthat for any area on the electrode weld face that could contact thesheet or workpiece surface to be welded, a line extending normal to theelectrode surface (normal direction) points into the plane of the sheetor workpiece surface to be welded. This is particularly important withrespect to electrode face protrusions that penetrate into a workpiecesurface and electrode face intrusions into which workpiece materialenters. In one embodiment, elements of the electrode face may beanalyzed by visualizing lines that extend outwardly (away from theworkpiece-contacting face of the protrusion or intrusion) and normal(perpendicular) to the face surface and determining how the normal lineintersects the workpiece surface that the elements will engage.

For electrode surface portions that are 90 degrees to the sheet orworkpiece surface, the normal direction would be parallel or at an angleof zero (0) degrees to the sheet or workpiece surface. Ideally, for allareas of the electrode surface the outward normal direction (pointingaway from the protrusion or intrusion solid surface) should be at least3 degrees below the sheet or workpiece surface. Areas of the electrodesurface where the outward normal direction points away from the plane(the outward normal points above the workpiece surface that theelectrode surface element is intended to engage or penetrate) of thesheet or workpiece surface would tend to be trapped during welding. Thisis illustrated in FIG. 3A. In FIG. 3A the surface 240 of a protrusion218 has a preferred contact angle with the outward normal direction 241pointing downwardly into the sheet surface and its protrusion surface242 has a non-preferred contact angle with its outward normal direction243 pointing upwardly and away from the surface of sheet 214. In FIG. 4,the surfaces 340 of intrusions 318 have non-preferred contact angles. Anoutward normal direction of a surface 340 would be difficult toillustrate in FIG. 4 but it would point outwardly from the workpiecesurface 314, 315. Intrusion surfaces 342 of intrusions 318 havepreferred contact angles. In the non-preferred angles of surfaces 340 ofFIG. 4, it is seen that when an electrode is withdrawn the hot softmetal would tend to be pulled against the protrusion or intrusion of theelectrode face.

The desired (or preferred) angles of a protrusion or intrusion shouldnot be too close to perpendicular to the workpiece surface, i.e., normaldirection near 0 degrees to the sheet or workpiece surface since wallsvertical to the sheet surface might promote sticking. In addition, sharpcorners in the protrusion/intrusions may promote sticking, roundedcorners would be preferred.

A third requirement is that the electrode weld face is shaped withprotrusion and/or intrusion features to allow it to perform its primaryfunction, i.e., produce structural welds in the sheet material.Excessively deep intrusions and especially protrusions on the electrodesurface may possibly have deleterious effects on weld formation.Electrode weld faces, and particularly those designed to spot weldaluminum, are designed to control the current density during the weldingprocess. Radiused electrode weld faces are used to provide a highinitial current density by providing a small contact area between thesheet surface and electrode. Compromising this area by placingintrusions on the weld face should have a small effect as long as mostof the contacting area is left unmodified. No more than 50% of thecontacting area should be modified by intrusions; preferably this shouldbe less than 20%. Protrusions, however, will make contact with the sheetbefore the remainder of the weld face. This can dramatically alter theinitial current density during the welding process. Intentionalprogramming of low initial current levels may be required for somegeometries to prevent overheating of the protruding electrode features.Once the protrusions have heated the sheet locally and penetrated thesurface, normal welding currents can be used. The amount of areamodified by the protrusions should be similar to that for intrusions, atmost 50% of the initial contact area and preferably less than 20%. Wehave found that for aluminum spot welding, in particular, more reliableweld current concentration and weld initiation can be obtained bydesigning the electrode such that the central contacting area of theradiused weld face is free from intrusions/protrusions and, prior tocurrent flow, contacts the sheet surface without additional contact fromnearby intrusions/protrusions. This design also extends electrode lifeby limiting excessive current flow through the intrusion/protrusionfeatures. Tests have shown that a 2.8 mm diameter area at the center ofthe electrode's contact area provides good weld performance.

In order to produce a visible feature in the welded surface theprotrusions may require a minimum height of about ten microns above theface of the electrode. Similarly, intrusions in the face of an electrodemay require a minimum depth of about ten microns. These height and depthdimensions may be larger depending on the thicknesses of the workpiecesas described above.

The formation of attractive weld surfaces must not compromise structuralweld integrity. Structural weld quality is determined by the weldingparameters of force, time, and current, but is also closely linked tothe geometry of the electrode weld face. Electrode weld faces aretypically either flat or convex for welding of both steel and aluminum.For steel welding the electrode weld face should be a minimum of 10%greater than the target weld size which is 4×(t)^(1/2) where t is thegoverning metal thickness, i.e., the thinner sheet in a two-stack weld.The weld face can be as large as the diameter of the electrode, whichtypically varies from about 12 mm to about 20 mm. Curvature of the weldface can vary dramatically from flat to 6 mm in radius. Smooth-facedelectrodes are typically used for steel spot welding. For aluminum spotwelding the electrode weld face should be a minimum of 2 mm greater thanthe target weld size of 4×(t)^(1/2) where t is the governing metalthickness. The weld face can be as large as the diameter of theelectrode, which varies from about 16 mm to about 19 mm. Curvature ofthe weld face is more limited. It can vary from flat to a 50 mm radiusof curvature for smooth-faced electrodes, i.e., roughness of less than 1micron. For textured electrodes with a surface roughness greater than 1micron and preferably greater than 5 microns the radius of curvature canvary from flat to 20 mm.

For spot welding of both aluminum alloys and steel materials, electrodeswith flat weld faces leave indentations on the surface of the sheet thathave a flat central region. If this region is not aligned carefully withthe rest of the sheet then it appears off-angle and can degradeperceived quality. To prevent this from occurring, electrodes withradiused weld faces are preferred for producing high perceived qualitywelds. These would produce imprints with an overall dish-shapedappearance in the material.

An example of high perceived quality welds formed in zinc coated steelsheet are shown in FIG. 5. In this example the mirror images of theletters “GM” were formed as intrusions on the face of a weldingelectrode and used in forming a sequence of two spot welds.

FIG. 6 illustrates a weld electrode face 622 with protrusions 618 in theshape of a snow flake or star.

FIG. 7 illustrates a weld electrode face 722 with protrusions 718 in theshape of a Chevrolet “Bow Tie” registered trademark.

Practices of the invention have been disclosed in term of someillustrative embodiments which are not intended to limit the scope ofthe invention.

1. A method of forming one or more electrical resistance spot welds inan assembly of overlying sheet metal layers, the method comprising:forming a pair of welding electrodes, the electrodes having weldingfaces for engaging opposing outer surfaces of an assembly of overlyingsheet metal layers, the welding faces being sized and shaped for formingat least one predetermined electrical resistance spot weld for joiningthe overlying sheets at a spot weld site, and at least one of thewelding faces having a protrusion or intrusion in the welding face forforming an attractive weld site image in the surface of at least onesheet contacted by the welding electrode face; and pressing the weldingelectrodes against opposing outer surfaces of the overlying sheet metallayers while passing an electrical current between the electrodesthrough the sheets to form a welding nugget between the sheets and todeform the outer surface of at least one of the sheets with theattractive weld site image formed as the obverse shape of the protrusionor intrusion of the electrode face.
 2. A method of forming one or moreelectrical resistance spot welds as recited in claim 1 in which theoverlying sheet metal layers are formed of an aluminum alloy and theimage is deformed in the surface of an aluminum alloy sheet.
 3. A methodof forming one or more electrical resistance spot welds as recited inclaim 1 in which the overlying sheet metal layers are formed of a steelalloy and the image is formed in the surface of a steel alloy sheet. 4.A method of forming one or more electrical resistance spot welds asrecited in claim 1 in which the overlying sheet metal layers are formedof a zinc coated steel alloy and the image is formed in the surface of azinc coated steel alloy sheet by a process comprising melting andre-solidifying zinc coating material.
 5. A method of forming one or moreelectrical resistance spot welds as recited in claim 1 in which theimage is in the form of a closed geometrical figure.
 6. A method offorming one or more electrical resistance spot welds as recited in claim1 in which the image is in the form of one or more alphanumeric figures.7. A method of forming one or more electrical resistance spot welds asrecited in claim 1 in which the height of any protrusion from the weldface or the depth of any intrusion in the weld face does not exceedabout fifty percent of the thickness of a sheet metal layer contacted bythe weld face.
 8. A method of forming one or more electrical resistancespot welds as recited in claim 1 in which the height of any protrusionfrom the weld face or the depth of any intrusion in the weld face doesnot exceed about twenty percent of the thickness of a sheet metal layercontacted by the weld face.
 9. A method of forming one or moreelectrical resistance spot welds as recited in claim 1 in which the areaof a protrusion or intrusion in the weld face extends outside the areaof a weld nugget formed between the sheet metal workpieces.
 10. A methodof forming one or more electrical resistance spot welds as recited inclaim 1 in which the area of a protrusion or intrusion in the weld faceis less than about fifty percent of the contact area of the weld face.11. A method of forming one or more electrical resistance spot welds asrecited in claim 1 in which the area of a protrusion or intrusion in theweld face is less than about twenty percent of the contact area of theweld face.
 12. A method of forming one or more electrical resistancespot welds as recited in claim 1 in which the weld image is formed in asurface of the welded assembly that is intended to be visible to a userof the assembly.
 13. A method of forming one or more electricalresistance spot welds as recited in claim 1 in which the weld image isformed in a surface of a welded assembly that is to be a member of anautomotive vehicle body.
 14. A method of forming one or more electricalresistance spot welds as recited in claim 1 in which the weld image isformed in a surface of a welded assembly that is to be a closure memberof an automotive vehicle body.
 15. A method of forming one or moreelectrical resistance spot welds as recited in claim 1 in which theheight of any protrusion from the weld face or the depth of anyintrusion in the weld face is at least about ten microns.
 16. A methodof forming one or more electrical resistance spot welds as recited inclaim 1 in which a weld face is round in plan view and convexly domedsuch that the center of the round domed face is intended to first engagea workpiece to be welded, the weld face comprising a protrusion and theprotrusion in the welding face being located a distance from the centerof the welding face to prevent overheating of the protrusion duringengagement of the protrusion with a workpiece during welding currentflow.
 17. A method of forming one or more electrical resistance spotwelds as recited in claim 16 in which workpiece is an aluminum alloysheet.
 18. A method of forming one or more electrical resistance spotwelds as recited in claim 1 in which surfaces of each protrusion orintrusion in the welding face which are intended to contact a workpiecesurface are shaped with a contact angle to minimize retention ofworkpiece material on such weld face surfaces.